1. Field of the Invention
The invention relates to a composite having two or more layers and including a barrier layer made from EVOH, and also including a layer made from a molding composition which includes a polyamide blend. The invention further relates to a molding composition of this type which includes a polyamide copolymer.
2. Discussion of the Background
When composites haying two or more layers are developed, for example, for use as a tube for carrying liquid or gaseous media in motor vehicles, the molding compositions must have sufficient chemical resistance to the media to be carried, and the tubes must meet all the mechanical requirements placed upon them, even after long exposure to fuels, oils or heat. In addition to meeting the requirement for adequate fuel resistance, the automotive industry demands improved barrier action from fuel piping, in order to reduce emissions of hydrocarbons within the environment. This has led to the development of tube systems having two or more layers, for example using EVOH as barrier layer material. However, EVOH is incompatible with PA11, PA12, PA612, PA1012 and PA1212, which can be used for the outer layer since they have good mechanical properties, good water absorption performance, and low susceptibility to environmental effects. It is therefore impossible to obtain the adhesion between the two layers that is indispensable for the application.
However, EVOH is compatible with PA6, PA66, PA6/66, and with maleic-anhydride-functionalized polyolefins. Molding compositions based on polymers of this type are, however, unsuitable as outer layer material.
DE-C 40 01 125 describes a motor vehicle pipeline composed of a tubular outer layer made from PA11 or PA12, an intermediate layer made from PA6, a barrier layer made from EVOH, and an inner layer made from PA6. A thin adhesion-promoter layer made from polyethylene or polypropylene, these being functionalized with maleic anhydride, is arranged between the outer layer and the intermediate layer.
A similar system is described in DE-C 40 01 126, where the motor vehicle pipeline is composed of a tubular outer layer made from PA11 or PA12, of a barrier layer made from EVOH, and of an adhesion-promoter layer situated between these and made from polyethylene or polypropylene, these being maleic-anhydride-functionalized.
However, the automotive industry has for some time required increased stability at relatively high temperature. This excludes solutions which include a polyolefin layer, since these have low heat resistance.
It has also been found ta prolonged contact with aqueous liquids or alcohol-containing fuel, in particular with heat, rapidly reduces the layer adhesion between functionalized polyolefin and polyamide, and this adhesion finally falls to values which are unacceptable in industry. The cause of this is thought to be alcoholysis or hydrolysis reactions.